from the GNU version of the Collaborative International Dictionary of English

n. One who engages in any contest; especially one who in ancient times contended in single combat in behalf of another's honor or rights; or one who now acts or speaks in behalf of a person or a cause; a defender; an advocate; a hero.

n. One who by defeating all rivals, has obtained an acknowledged supremacy in any branch of athletics or game of skill, and is ready to contend with any rival.

transitive v. To furnish with a champion; to attend or defend as champion; to support or maintain; to protect.

from The Century Dictionary and Cyclopedia

n. One who undertakes to defend any cause; especially, one who engages in combat or contention in behalf of another, or in any representative capacity: as, the champion of an army or of a party; a champion for the truth, or of innocence.

n. More generally, a hero; a brave warrior.

n. One who has demonstrated his superiority to all others in some matter decided by public, contest or competition, as prize-fighting, pedestrianism, rowing, plowing, etc.

First among all competitors or contestants: as, a champion oarsman.

Hence By extension, of the first rank or highest excellence in any respect; unexcelled.

To maintain or support by contest or advocacy; act as champion for.

See champian.

n. An animal or a plant to which, or to its owner, the first prize has been awarded in any general competition, as at a horse-, cattle-, bench-, or other show, or which has taken a number of prizes.

n. Some variety of vegetable, fruit, implement, or the like, for which the highest excellence is claimed.

Etymologies

Middle English champioun, combatant, athlete, from Old French champion, from Medieval Latin campiō, campiōn-, from Latin campus, field.

(American Heritage® Dictionary of the English Language, Fourth Edition)

From Middle English, from Old French, from Medieval Latin campio ("a champion, combatant in a duel"), from campus ("a battle, duel"), from Latin campus ("a field, a plain, a place of action"). (Wiktionary)

Examples

Via a bloodthirsty class warrior Democratic flack: "A real estate lawyer, Cantor earned the nickname 'champion of the overdog' in the General Assembly for legislation such as a bill to give fuel tax refunds for pleasure boats."

Fred ♪ ♫ ♪ says: maybe the states rights troll can tell us who their champion is as the republicans under bush overrode states rights in California over possession of weed. federal law took prescedent and people went to jail who were obeying the laws of their state.